Leukemia is a malignant disease of the blood-forming organs, which involves the distorted proliferation and development of white blood cells in bone marrow and blood. Leukemias are usually classified as myelogenous or lymphocytic, according to the types of cells that are involved. Within these groups, there are chronic and acute conditions, which vary in duration and character. Leukemias tend to have age specificity, for example acute lymphoid leukemia generally occurs in young children, while acute myelogenous leukemia is found principally in young adults.
The ability to isolate purified populations of hematopoietic stem cells and myeloid progenitors based on characteristic cell surface (phenotypic) markers has made it possible to identify genes involved in hematopoietic stem cell self-renewal. Normal hematopoietic stem cells, unlike committed hematopoietic progenitors, have the capacity to divide and make identical progeny without undergoing differentiation i.e. self-renewal. Deregulation of self-renewal pathways, which are normally tightly regulated in hematopoietic stem cells, has recently been recognized as an important step in leukemic progression.
Chronic myelogenous leukemia (CML) is a disease having distinct clinical and pathological features. The cause of CML is a specific chromosomal translocation between human chromosomes 9 and 22, resulting in a product commonly referred to as the Philadelphia chromosome. The gene for the tyrosine kinase c-abl resides on the distal arm of human chromosome 9, while the gene for c-bcr resides on human chromosome 22. The translocation places the promoter distal three exons of ABL, including those elements that encode the tyrosine kinase domain, downstream of either the first or second exon of BCR. This chimeric gene, BCR-ABL, encodes a fusion protein often referred to as p185bcr-abl or p210bcr-abl, depending upon the inclusion of the second exon of BCR. p185bcr-abl causes acute leukemia, typically lymphoblastic; p210bcr-abl usually causes CML which may progress to myeloid or lymphoid blast crisis.
Treatment of leukemias has traditionally relied on chemotherapy using anti-neoplastic agents, radiation therapy, corticosteroid therapy and immunotherapy, which may be performed in combination with transplantation of hematopoietic stem cells. Different therapies are utilized depending upon the type of leukemia being treated.
Recently a new class of antiproliferative agents called signal transduction inhibitors has been introduced, which interferes with the pathways that signal the growth of tumor cells. Gleevec (imatinib mesylate) is targeted to the constitutively active abnormal tyrosine kinase created by the Philadelphia chromosome. Imatinib is also an inhibitor of the receptor tyrosine kinases for platelet-derived growth factor (PDGF) and stem cell factor (SCF), c-kit, and inhibits PDGF- and SCF-mediated cellular events.
However, despite the effectiveness of imatinib in inducing both hematologic and cytogenetic remissions in the majority of chronic phase (CP) CML patients, some patients progress, in part as a result of amplification of BCR-ABL and point mutations in the binding site for imatinib on the abl tyrosine kinase active site of P210. In addition, patients who become resistant to Imatinib and develop accelerated phase (AP) or blast crisis (BC) frequently demonstrate clonal evolution with trisomy 8 and other chromosomal abnormalities suggesting that activation of other oncogenes may contribute to disease progression. Finally, the role of BCR-ABL amplification and additional oncogene activation in HSCs or more committed progenitors with increased proliferative and self-renewal capacity, as a result of aberrant overexpression of hematopoietic stem cell self-renewal genes such as β-catenin, is of great interest.
In another myeloid leukemia, t(8;21) acute myelogenous leukemia, marrow from patients in complete remission contains apparently normal hematopoietic stem cells that produce AML1-ETO transcripts, and their presence during remission implies that such hematopoietic stem cells are pre-leukemic rather than leukemic cells (these transcripts participate in the development of acute myeloid leukemia; AML1-ETO is formed by the fusion of part of the AML1 gene on chromosome 8 with part of the ETO gene on chromosome 21). Similarly, genomic BCR-ABL persists in the marrow of some CML patients who are in a sustained complete cytogenetic remission, and has been detected at very low levels in the leukocytes of healthy individuals, which suggests that pre-leukemic hematopoietic stem cells or more differentiated progenitor cells need additional mutations for progression to overt leukemia.
Bone marrow HSCs are functionally defined by their unique capacity to self-renew and to differentiate to produce all mature blood cell types. In general, the process of development from pluripotent progenitors to mature cells with specific functions involves the progressive loss of developmental potential to other lineages. A hierarchy has emerged in which each successive developmental stage loses the potential to become a specific cell type or class of cells. This stepwise developmental process has been considered linear in the sense that once a cell has made a developmental choice it cannot revert. The earliest known lymphoid-restricted cell in adult mouse bone marrow is the common lymphocyte progenitor (CLP), and the earliest known myeloid-restricted cell is the common myeloid progenitor (CMP). Importantly, these cell populations possess an extremely high level of lineage fidelity in in vitro and in vivo developmental assays. A complete description of these cell subsets may be found in Akashi et al. (2000) Nature 404(6774):193, U.S. Pat. No. 6,465,247; and published application U.S. Ser. No. 09/956,279 (common myeloid progenitor); Kondo et al. (1997) Cell 91(5):661-7, and International application WO99/10478 (common lymphoid progenitor); and is reviewed by Kondo et al. (2003) Annu Rev Immunol. 21:759-806, each of which is herein specifically incorporated by reference.
CD34+ cells harbor virtually all in vitro clonogenic potential; however, the CD34+ population is heterogeneous. Only a small fraction (1-10%) of CD34+ cells that do not express mature lineage markers (Lin−, including the markers CD3, CD4, CD8, CD19, CD20, CD56, CD11b, CD14, and CD15) have multilineage (lymphoid and myeloid) developmental potential. The majority of CD34+ cells (90-99%) coexpress the CD38 antigen, and this subset contains most of the lineage-restricted progenitors.